Studies are being continued concerning the mechanism of plasma membrane passage of inhibitors of dihydrofolate reductase of diaminopyrimidine type. The enzymatic and cellular basis which is responsible for the greater sensitivity of mouse cells, as compared to human cells to 5-fluoropyrimidines is being investigated. BIBLIOGRAPHIC REFERENCES: Laskin, J.D. and Hakala, M.T. Metabolism of 5-Fluorouracil in Mouse and Human Cells as a Basis for Their Differing Sensitivity. Proc. Am. Assoc. Cancer Res. 18, 57, 1977. Greco, W.R. and Hakala, M.T. The Uptake of a Lipophilic Antifolate, 2,4,-Diamino-5-adamantyl-6-methylpyrimidine, by Human Cells. Feder. Proc., 36, 336, 1977.